Engines, including diesel engines, gasoline engines, gaseous fuel-powered engines, and other engines known in the art exhaust a complex mixture of emissions. These emissions may be composed of gaseous compounds, including NO, NO2, CO, CO2, unburned hydrocarbons CxHy, water vapor, O2, and solid carbon particulate matter also known as soot.
The amount of gaseous compounds emitted to the atmosphere from an engine may be regulated depending on the type of engine, size of engine, and/or class of engine. A method implemented by engine manufacturers to comply with the regulation of particulate matter exhausted to the environment includes removing the particulate matter from the exhaust flow of an engine with a device called a particulate trap. Another method that has been implemented by engine manufacturers to comply with the regulation of engine emissions has been to employ various catalysts to purify the exhaust gas from the engine before emitting the gas to the atmosphere. Most catalysts either employ conventional lean NOx mechanisms or on-board ammonia production by urea hydrolysis, known as urea selective catalytic reduction. These mechanisms seek to convert harmful NOx into innocuous constituents such as N2, CO2, and water, and to neutralize or eliminate NH3 slip attributed to the urea selective catalytic reduction aftertreatment system.
One example of utilizing lean NOx mechanisms to purify exhaust emissions is described in U.S. Pat. No. 6,378,298 (the '298 patent) issued to Harima et al. on Apr. 30, 2002. The '298 patent utilizes a conventional lean NOx catalyst to purify the exhaust gas. Specifically, the '298 patent describes a split exhaust flow including a first passageway and a second passageway that both allow exhaust gas to flow from the engine to the catalyst. The '298 patent further discloses a flow controller that controls the amount of exhaust gas flowing from the engine through the first and second passageways based on a temperature of the exhaust gas. The first passageway contains an accelerated cooling portion having a cross section designed to allow a large release of heat from the exhaust gas in the first passageway. The second passageway has a cross section designed to allow a small release of heat from the exhaust gas in the second passageway. The amount of exhaust gas flowing through the first passageway is made larger than that through the second passageway when the temperature of exhaust gas emitted from the engine is to be lowered by a relatively large degree before reaching the catalyst. In contrast, the amount of exhaust gas flowing through the second passageway is made larger than that through the first passageway when the temperature of exhaust gas emitted from the engine is to be lowered by only a small degree before reaching the catalyst.
Although the exhaust purifying system of the '298 patent may reduce the amount of NOx gas exhausted to the environment, it relies exclusively on lean NOx mechanisms to purify exhaust emissions. Thus, the system described in the '298 patent will be ineffective in situations where lean NOx mechanisms alone cannot sufficiently convert harmful emissions into innocuous constituents.
This disclosure is directed to overcoming one or more of the problems set forth above.